Blood platelets are a key component of the human body's response to injury, whether from external trauma or internal ailments, including diseases from heart disease to cancer. In response to injury or illness, blood platelets become activated in varying numbers and to varying degrees, ranging from fully at rest to completely activated and bound to a surface or other cell. The presence and prevalence of activated platelets, as defined by their morphologies and receptor states, have been associated with important diseases, including coronary heart disease (CHD) and congestive heart failure (CHF). Further, platelet activation statistics (information on platelet activation states) could offer a highly informative hemostatic indicator during procedures that particularly stress the circulatory system, including abdominal aortic aneurysm repair and coronary bypass surgery. While flow cytometry can characterize the activation states of a population of platelets, cytometers are expensive, cumbersome laboratory instruments ill- suited to real-time or point-of-care diagnosis. A rapid assay of platelet activation could enable routine screening for early warning of impending vascular thrombotic diseases, titration of anti-platelet therapy in CHF, monitoring in clinical settings, quality control in blood centers, and would also provide considerable utility as a research tool to further study links between platelet activation state and disease. We propose a method to measure platelet activation using a chip-based flow cytometer with microelectrodes for impedance analysis of single platelets. Such an instrument will simultaneously provide information regarding cell count and activation states given by the size and dielectric properties of the cells. The key hypothesis is that platelet impedance characteristics indicate their activation state. If this proves correct, the benefits include high speed, automation, low cost, and very compact instrument designs. Importantly, the assay will require only small samples of blood (microliters). The use of impedance spectroscopy will make this a label-free assay (no binding required to receptors that might affect platelet activation state). The integration of on-chip hydrodynamic focusing will lead to increased resolution and reproducibility of the impedance spectrum. Holding the platelets within a core flow surrounded by a sheath flow will allow not only for drastic reduction in shear forces, but also for focusing of the electronic probe signal through differences in core/sheath conductivities and dielectric coefficients.